1. Field of the Invention
This invention relates to a slippage control system for an automotive vehicle.
2. Description of Related Art
In recent years, automotive vehicles have been equipped with slippage control systems called traction control devices, in which wheel drive torque is reduced during acceleration so as to prevent excessive slippage of the driving wheels on the road surface. The reduction of wheel drive torque ordinarily results from at least one of a reduction in engine torque and a reduction in brake force applied to the drive wheels. Such slippage control is accomplished through the implementation of changes which correspond to changes in a road surface friction factor or resistance. For instance, a threshold value of slippage is changed so as to conduct or initiate the slippage control more easily as the road friction factor .mu. becomes lower or, otherwise, a target value of slippage of the drive wheels is made lower as the road friction factor .mu. becomes lower.
In this regard, changes in the nature of slippage control require the estimations of road surface friction factors, for which there are a number of estimation methods. For example, Japanese Unexamined Patent Publication 60-99757 reveals the estimation of a road surface friction factor made on the basis of driven wheel speed while a slippage control is in effect.
Heretofore, while estimating road surface resistance, an accurate estimation of road surface resistance is very difficult. A favorable, accurate estimation of road surface resistance with high responsiveness has been developed by conducting an estimation of road surface resistance on the basis of rectilinear gravitational acceleration of the vehicle during the implementation of vehicle slippage control. However, it has been also found that, depending upon vehicle running conditions, great errors may occur in the estimation of road surface resistance based on rectilinear gravitational acceleration of the vehicle.
Specifically, considering that the vehicle ordinarily turns circularly on a road which has a high surface resistance, since rectilinear gravitational acceleration acting on the vehicle may be zero (0), or close to zero (0), the road must be estimated to have a low surface resistance. Therefore, the drive wheel torque is considerably reduced, as being unnecessary. This results in a feeling of stalling or speed dropping.